Underfeather

Underfeather is a term used in ornithology to describe the layer of soft, small feathers that lies beneath a bird's outer contour feathers. This underlayer forms part of the plumage known as the down or undercoat and is mainly composed of plumulaceous, fluffy feathers that trap air close to the body to provide insulation.

The underfeather includes down feathers that are short and fluffy, densely packed to maximize insulation. In some species, other light, soft underlayers may be present as well. The density and length of the underfeather vary with climate and lifestyle; Arctic and other cold-adapted birds typically have thicker underfeathers, while tropical birds possess a lighter undercoat.

The primary function of the underfeather is thermoregulation, reducing heat loss in cold environments by trapping still air. It also contributes to the overall plumage condition and can aid buoyancy when birds are in water, in concert with the outer feathers and preen oil that maintains feather properties.

Development and molts: Underfeathers are renewed during molt, usually in coordination with the shedding and regrowth of outer contour feathers. The timing and pattern of molt are species-specific and can be influenced by season and climate.

Related topics include plumage, down feathers, molt, and thermoregulation in birds.